1. Field of the Invention
The invention relates to a control apparatus and a control method for a belt-type continuously variable transmission having a pair of variable effective diameter pulleys that are actuated by hydraulic cylinders and a transmission belt wound around the pulleys. In particular, the invention relates to a technology for setting the base pressure for the hydraulic pressure of each hydraulic cylinder.
2. Description of the Related Art
As is widely known, a control apparatus for controlling a belt-type continuously variable transmission having a pair of variable effective diameter pulleys that are actuated by hydraulic cylinders and a transmission belt wound around the pulleys needs to set the base pressure for the hydraulic pressure of each hydraulic cylinder appropriately.
For example, JP2728466 describes a hydraulic pressure control apparatus for a hydraulically-driven transmission. The transmission recited in this patent is a continuously variable transmission that has a primary pulley, a secondary pulley, and a belt wound around the primary and secondary pulleys. The speed ratio of the continuously variable transmission is controlled by operating the primary pulley so as to archive the target speed ratio. At this time, the force for clamping the belt (belt clamping force) is controlled by operating the secondary pulley so as to prevent slipping between the primary pulley and the belt and slipping between the secondary pulley and the belt. The hydraulic pressure control apparatus descried controls the line hydraulic pressure that is the base pressure for the hydraulic pressure of the hydraulic chamber for the primary pulley (will hereinafter be referred to as “primary hydraulic pressure”) and the hydraulic pressure of the hydraulic chamber for the secondary pulley (will hereinafter be referred to as “secondary hydraulic pressure”) based on the input torque and the speed ratio of the continuously variable transmission. That is, in this way, the hydraulic pressure control apparatus improves the shift response, prevents slipping of the belt, and increases the power transmission efficiency.
The hydraulic pressure control apparatus has a hydraulic circuit that supplies the line hydraulic pressure directly to the secondary-pulley side hydraulic chamber, and thus the line hydraulic pressure and the secondary hydraulic pressure are substantially equal to each other. Therefore, although the line hydraulic pressure can be set to a level that is required to change the width of the groove of the secondary pulley appropriately, there is a possibility that the line hydraulic pressure fails to be set to a level required for the primary hydraulic pressure, and it may deteriorate the shift response of the continuously variable transmission.
To cope with this, one option is to have a hydraulic circuit that through which the line hydraulic pressure can be controlled independent of the primary and secondary hydraulic pressures and control the line hydraulic pressure based on the primary and secondary hydraulic pressures. In general, in a belt-type continuously variable transmission, the secondary hydraulic pressure is directly adjusted to the target value while the primary hydraulic pressure is built up as a result of the continuously variable transmission being shifted and the belt clamping pressure being produced through the control of the flow rate of hydraulic fluid to and from the primary pulley side hydraulic chamber. As such, when setting the line hydraulic pressure, it is necessary to estimate the value of the primary hydraulic pressure and use it for the setting of the line hydraulic pressure, while the set value of the secondary hydraulic pressure is used as it is.
The primary hydraulic pressure is a hydraulic pressure that is needed to maintain the speed ratio constant and to change the speed ratio. The primary hydraulic pressure can be calculated based on, for example, the belt clamping pressure, the speed ratio, and the rate of change in the speed ratio (the speed ratio changing rate). For example, when the continuously variable transmission is to be shifted up by supplying the hydraulic fluid to the primary-pulley side hydraulic chamber, the value of the primary hydraulic pressure that is needed to reduce the width of the groove of the primary pulley as required is calculated by adding a required shift pressure, which is the hydraulic pressure for producing the thrust force for changing the groove width of the primary pulley to change the speed ratio, to a steady balance pressure, which is the hydraulic pressure for maintaining the speed ratio constant.
In the case where the required shift pressure is calculated using the rate of change in the target speed ratio as the speed ratio changing rate, when the actual speed ratio is lagging behind the target speed ratio while the continuously variable transmission is being shifted up, the required shift pressure for the upshift may be zeroed despite the fact that the upshift of the continuously variable transmission is sill ongoing. In this case, the hydraulic pressure for the upshift is not sufficient, and this may reduce the followability to the target speed ratio and thus delay the completion of the upshift.
On the other hand, in the case where the required shift pressure is calculated using the rate of change in the actual speed ratio as the speed ratio changing rate, when the actual speed ratio is lagging behind the target speed ratio while the continuously variable transmission is being shifted up, the required shift pressure for the upshift may be made relatively small in calculation. In this case, too, the hydraulic pressure for the upshift is not sufficient, and this may reduce the followability to the target speed ratio and thus the shift response.
In order to avoid such reduction of the followability to the target speed ratio, the line hydraulic pressure needs to be set high with respect to the required primary hydraulic pressure. In this case, however, the line hydraulic pressure may be made higher than necessary, and it may reduce the fuel economy.